Question

"(1)Why is DNA replication continuous and discontinuous in a replication fork?
(2)Explain the imporrtance of origin of replication in a replication fork?"


class 12th CBSE Biology sample paper

Answer 1

(1) For long DNA molecules, since the two strands of DNA cannot be separated in its entire length (due to very high energy requirement), the replication occur within a small opening of the DNA helix, referred to as replication fork. The DNA-dependent DNA polymerases catalyse polymerisation only in one direction, that is 5'→3'. This creates some additional complications at the replicating fork. Consequently, on one strand (the template with polarity 3'→5'), the replication is continuous, while on the other (the template with polarity 5'→3'), it is discontinuous. The discontinuously synthesised fragments are later joined by the enzyme DNA ligase .

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(2)  The DNA polymerases on their own cannot initiate the process of replication.  Also the replication does not initiate randomly at any place in DNA. There is a definite region in E. coli DNA where the replication originates. Such regions are termed as origin of replication.

 It is because of the requirement of the origin of replication that a piece of DNA if needed to be propagated during recombinant DNA procedures, requires a vector. The vectors provide the origin of replication.

Answer 2

(1) Due to difference in polarity of DNA strand  

(2) The origin of replication is important to start the replication.

Explanation:

• In DNA one strand is in 5' to 3' direction and another strand is in 3' to 5' direction. The DNA polymerase synthesize the new strand in 5' to 3' direction so one strand is synthesized continuously and other discontinuously.
• The strand which is synthesized regularly is called the leading strand and the strand which is synthesized in a discontinuous manner is called the lagging strand.
• The origin of replication is the site in DNA from where the replication starts. It is AT-rich region. So it is important to start replication in the direction of the replication fork.

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